The present invention relates to a television receiver for teletext for receiving a composite video signal having text information multiplexed on a television signal, and more particularly to a television receiver for teletext for displaying television video signal and teletext signal simultaneously on a picture screen by superimposing or dividing the screen.
Further particularly it relates to a television receiver for teletext for displaying a television video signal and teletext signal simultaneously on a screen of aspect ratio of 16:9 known as wide television.
FIG. 1 shows a schematic constitution of a television receiver for teletext of a prior art. In the diagram, reference numeral 11 is an input signal of demodulated composite video signal, 12 is a television video signal processing circuit for receiving a composite video signal and converting into a television signal a (signal of three primaries R, G, B), 13 is a teletext signal receiving circuit for extracting a teletext signal superposed in a vertical blanking period of input composite video signal to convert into teletext video signal b (signal of three primaries R, G, B), and generating a composite control signal (YS signal ) c synchronized with the teletext video signal b, 14 is a synthesizing circuit for superimposing a teletext video signal b from the teletext signal receiving circuit 13 for the television video signal a from the television video signal processing circuit 12 on the basis of the composite control signal (YS signal) c from the teletext signal receiving circuit 13, and 15 is a display device such as CRT and LCD for displaying the composite video signal d.
The operation is described below. FIG. 2(a) shows a television video signal a in part of one horizontal line, and FIG. 2(b) shows a teletext video signal b in part of one horizontal line. FIG. 2(c) shows a composite control signal (YS signal) c generated completely at same timing (in same time width) synchronously with the teletext video signal b.
The teletext video signal b, for example, at the timing of horizontal line 16 of letter xe2x80x9clxe2x80x9d in FIG. 3(a), produces two waveforms as shown in FIG. 3(b). and the composite control signal (YS signal) c is generated at the same timing as the teletext video signal b as shown in FIG. 3(c).
FIG. 2(d) shows the composite video signal d having the television video signal a and teletext video signal b synthesized according to the composite control signal (YS signal) c. In the composite video signal d, the television video signal a is extracted at the timing of teletext video signal b.
FIG. 4 shows the detail of the teletext signal receiving circuit 13 in the television receiver for teletext. In this diagram, reference numeral 31 is a teletext signal decoder for extracting and decoding the teletext signal superposed in the vertical blanking period from the input composite video signal, 61 is a memory for storing decoded teletext data, 62 is a memory control circuit for controlling writing and reading of the memory 61, 35 is a line memory for storing the teletext data for the portion of one horizontal line temporarily, 42 is a look-up table for converting the teletext data read out from the line memory 35 into color data of R, G, B, and 37 is a D/A converting circuit for converting the developed color data into analog video signal. Reference numeral 43 is an opaque color control signal generating circuit for receiving bit map data from the line memory 35, generating YM signal (opaque color control signal), and issuing into a video chroma signal processing circuit 23, and 44 is a changeover signal generating circuit of teletext signal/video chroma signal for receiving bit map data, generating YS signal (changeover signal of teletext signal/video chroma signal), and issuing into a switch 26. By changing over the switch by the YS signal, both video chroma signal and teletext signal are displayed simultaneously on the picture screen. In the case of superimposed broadcast of YM signal, the background of the script is opaque, and in the case of teletext, not superimposed broadcast, the text is displayed and accompanied by foreground color and background color.
Furthermore, FIG. 5 shows a constitution of a television receiver for teletext for explaining the prior art especially from the aspect of synchronizing signal. In this diagram, reference numeral 501 is an antenna, and 502 is a receiving circuit for receiving television broadcast wave and extracting a composite video signal of base band. The composite video signal from the receiving circuit 502 is supplied simultaneously also to a video chroma signal processing circuit 503, a teletext signal receiving circuit 504, and a synchronism separating circuit 505. The video chroma signal processing circuit 503 is a circuit for demodulating the composite video signal, and extracting color signals of three primaries R, G, B, and the teletext signal receiving circuit 504 is a circuit for extracting the teletext signal superposed on the composite video signal, and converting into color signals of three primaries R, G, B. Reference numeral 506 is a double speed converting circuit for compressing the image of R, G, B signals from the video chroma signal processing circuit 503 by xc2xd in the horizontal direction, and 507 is a double speed converting circuit for compressing the image of R, G. B signals from the teletext signal receiving circuit 504 by xc2xd in the horizontal direction. These double speed converting circuits 506, 507 are individually composed of three A/D converters for converting R. G, B signals into digital data, three line memories for storing the converted data and reading out at double speed of writing speed, and three D/A converters for converting the read data into analog R, G, B signals. Reference numeral 510 is a changeover circuit having a switch 508 and a switch 509. The switch 508 is for changing over the signal from the double speed converting circuit 506 and the signal from the double speed converting circuit 507 at high speed in order to display the television video signal and teletext signal simultaneously, and the switch 509 is for selectively changing over the signal from the video chroma signal processing circuit 503, the signal from the teletext signal receiving circuit 504 and the signal from the switch 508. Reference numeral 511 is a picture tube drive circuit, and 512 is a picture tube.
The switches 508, 509 in the changeover circuit 510 are designed to be changed over by a control signal from a controller 513. The horizontal synchronizing signal and vertical synchronizing signal separated in the synchronism separating circuit 505 are respectively supplied into a horizontal deflection circuit 514 and a vertical deflection circuit 515.
The signal generated by the double speed converting circuits 506, 507 are compressed by xc2xd in the horizontal direction. On a wide screen of the picture tube 512, when displaying both television video signal and teletext signal simultaneously, generally, the aspect ratio is 4:3 in both signals (see FIG. 6). Accordingly, in both signals, the screen must be compressed also in the vertical direction, and a vertical amplitude changeover circuit 516 and a vertical screen position control circuit 517 are provided, and their outputs are supplied into the vertical deflection circuit 515. The vertical amplitude changeover circuit 516 and vertical screen position control circuit 517 are designed to be controlled by the controller 513. The vertical amplitude changeover circuit 516 reduces the amplitude of the sawtooth wave supplied to the vertical deflection circuit 515. The vertical screen position control circuit 517 adds a direct-current voltage to the sawtooth wave reduced in amplitude, and the timing of the sawtooth wave is adjusted in the vertical direction.
As a result, as shown in FIG. 6, on the wide screen of the picture tube 512, a screen 512a of television video signal, and a screen 512b of teletext signal, both of aspect ratio 4:3, are displayed. At this time, the double speed converting circuits 506, 507 are changed over to high speed by the switch 508. Reference numeral 512c is a no-picture area without display. When the vertical positions of the both screens 512a, 512b are fixed, deterioration of phosphor in these portions is promoted, and the screen luminance is uneven in the case of full screen display. Accordingly, the vertical screen position control circuit 517 is operated and the vertical positions of the both screens 512a, 512b are changed appropriately as shown in FIG. 6(a) to (c).
Thus, in the prior art, if attempted to display the television video signal and teletext signal simultaneously on a same screen by overlapping, the composite control signal (YS signal) c is generated at the same timing and same time width as the teletext video signal b, and the television video signal a and teletext video signal b are synthesized by such composite control signal (YS signal) c, and therefore the boundary is not clear between each character of the teletext picture displayed in the display device 15 and the television picture in the immediate vicinity, and in particular when the luminance levels of the two are very close, it is hard to read the superimposed text.
Yet, the text is always accompanied by foreground color and background color, and the foreground color and background color change variously, and hence the displayed text itself is hard to see.
Further, on the wide screen with aspect ratio of 16:9, if attempted to display the television video signal and teletext signal simultaneously on the same screen by dividing the screen, the screen 512a of television video signal and screen 512b of teletext signal of aspect ratio 4:3 displayed by compressing in the horizontal direction and vertical direction are smaller than the full screen, and the no-picture area 512c is wide, and the full screen (wide screen) is not utilized effectively.
It is hence an object of the invention to display teletext signal more explicitly even when displaying the teletext signal on the picture screen together with television video signal, simultaneously by superimposing or by dividing the screen.
To achieve the object, the invention provides a television receiver for teletext comprising video signal receiving means for receiving a video signal superposing a teletext signal, teletext signal receiving means for extracting the superposed teletext signal from the composite video signal received by the video signal receiving means, preprocessing means for issuing a preprocessing correction signal on the basis of the teletext signal extracted from the teletext signal receiving means, synthesizing means for synthesizing the video signal received by the video signal receiving means, preprocessing correction signal issued from the preprocessing means, and teletext signal received by the teletext signal receiving means, and display means for displaying the output of the synthesizing means on a picture screen, wherein the boundary of the television video signal and teletext signal is more clear, and the text of the teletext signal is easy to read.
In particular, the preprocessing means generates a foreground color and a background color by the teletext data,decoded by the teletext signal receiving means, and issues a preprocessing correction signal for decreasing the luminance of the surrounding background color as compared with the text foreground color in the display region of the teletext signal of the picture screen, and the synthesizing means synthesizes.so as to display the video signal from the video signal receiving means in the background of the preprocessing correction signal, so that the boundary of the television video signal and teletext signal is clearer, and it is easier to read the text of the teletext signal.
Moreover, the preprocessing means generates the teletext data decoded by the teletext signal receiving means delayed or advanced by specific time in the horizontal scanning direction to obtain teletext data with phase deviation, and issues a preprocessing correction signal to be low luminance level signal for a specific time between the teletext data and teletext data with phase deviation, and the synthesizing means synthesizes to display by reducing the luminance before or after the text, in the display region of the teletext signal on the picture screen, and therefore the boundary of the television video signal and teletext signal is clearer, and it is easier to read the text of the teletext signal.
Still more, the preprocessing means displays the teletext data decoded by the teletext signal receiving means, either above or beneath the region of display of video signal by the video signal receiving means on the picture screen, and also issues a preprocessing correction signal for controlling so as to be displayed in the vertical scroll state, and the synthesizing means synthesizes the video signal and teletext data so as not to form a no-picture area on the picture screen, so that the television video signal and teletext signal can be displayed effectively on the screen by separating.